Field of the Invention
This invention relates to a power transmission used for vehicles provided with a continuously variable transmission (hereinafter called a "CVT").
Description of the Prior Art
A speed change ratio is selected in several steps directly through the shift lever operation of a driver in conventional manual transmission, and as a function of a throttle position and vehicle speed in conventional automatic transmissions. While an internal combustion engine is preferably run with a minimum specific fuel consumption, the speed change ratio in conventional transmissions is set in consideration of driveability in acceleration and by steps so that it was difficult to run the internal combustion engine with the minimum specific fuel consumption over the whole running range of the engine. Then, this applicant provided previously a power transmission for vehicles which can controllably run the internal combustion engine with minimum specific fuel consumption over the whole running range by the use of a CVT. Namely, according to the power transmission for vehicles in which the rotation of the internal combustion engine is transmitted to drive wheels through a CVT, the horsepower requirement of the internal combustion engine is determined as a function of operation amount of an accelerator pedal, and the output torque and speed of an engine which is run to achieve the horsepower requirement with the minimum specific fuel consumption are defined as a desired output torque and speed to feedback control the speed ratio of CVT so that the internal combustion engine speed becomes the desired one and feedback control the throttle position of an intake system through a throttle actuator so that the output torque of the internal combustion engine becomes the desired one. In conventional vehicles with stepped speed change gears, a throttle valve and an accelerator pedal are connected to an engine so as to have strict reliability, responsive property and accuracy, so that the precise operation of the throttle actuator is required, which provides an obstacle to the realization of said power transmission for vehicles.
Also, in a belt system CVT, engine power is transmitted from the input side pulley through a belt to the output side pulley and the pressing force of the output side pulley to the belt is controlled by line pressure supplied to the output side hydraulic cylinder. Though the line pressure should ensure the power transmission by avoiding the slip of the belt while being controlled to a minimum value necessary for restraining the drive loss of an oil pump, the line pressure is calculated as a function of only torque of the output side pulley in conventional line pressure controls so that insufficient control accuracy, secure power transmission and drive loss restraint of the oil pump are left unimproved.